1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) mobile communications system, specifically, to a base band signal transmitting device in the base station of a mobile communications system.
2. Description of the Prior Art
FIG. 1 is a block diagram of a transmitting device in the base station of a conventional mobile communications system. Referring to FIG. 1, the transmitting device includes: a base band signal transmitting circuit 1 for summing digital signals received from a plurality of voice channels and converting the summed digital signal into an analog signal; a base/intermediate frequency band upward conversion circuit 21, connected to base band signal transmitting circuit 1, for converting the base band analog signal into a signal of intermediate frequency band of 4.95 MHz; a cellular/PCS band upward conversion circuit 31, connected to base/intermediate frequency band upward conversion circuit 21, for converting the intermediate-frequency analog signal received from base/intermediate frequency band upward conversion circuit 21 into a signal of high-frequency band of 800 MHz or 1.8 GHz; a power amplifier 41, connected to cellular/PCS band upward conversion circuit 31, for amplifying the power of the high-frequency signal; and a transmission antenna 42, connected to power amplifier 41, for radiating the high-frequency signal.
FIG. 2 is a block diagram of a base band signal transmitting device in the base station of the conventional mobile communications system. Referring to FIG. 2, the base band signal transmitting device includes: four CDMA modems 52 for modulating digital voice signals received from a vocoder into direct band spread signals by sectors, to be suitable for CDMA standard, IS-95, to generate digital base band signals, and transmitting them in parallel; a digital summing device 53, commonly connected to CDMA modems 52, for summing the direct band spread digital signals from CDMA modems 52 by sectors to generate I-signal in in-phase and Q-signal in quadrature-phase, and transmitting them in parallel; a digital/analog converter (DAC) 54, connected to digital summing device 53, for converting the I- and Q-digital signals into analog signals; a base band low pass filter 55, connected to DAC 54, for filtering the high-frequency component from the analog signals from DAC 54, to be suitable for CDMA signal standard; an amplifier 56, connected to base band low pass filter 55, for amplifying the low-pass filtered CDMA analog signals; and a buffer amplifier 57, connected to amplifier 56, for adjusting the amplified CDMA analog signals, and performing the impedance matching between summing amplifiers of the base/intermediate band upward conversion circuit.
CDMA modems 52, digital summing device 53, DAC 54, base band low pass filter 55 and amplifier 56 are mounted on one board, buffer amplifier 57 is mounted on another board, and the base/intermediate frequency band upward conversion circuit including the summing amplifiers is mounted on another separate board. Buffer amplifier 57 is connected to the summing amplifier of the base/intermediate frequency band upward conversion circuit by sectors using a separate cable, and base band low pass filter 55 is composed of an inductor and capacitor.
The signal transmission procedure in the base band signal transmitting device in the base station of the above-described conventional mobile communications system is explained below. First, the voice digital signals generated by the vocoder are converted into direct band spread signals by sectors to become digital base band signals according to CDMA modems 52, and transmitted in parallel to digital summing device 53 which sums the digital base band signals by sectors. The digital base band signal summed by digital summing device 53 passes through digital/analog conversion step, unnecessary frequency component removing step and amplification step, and reaches the base/intermediate frequency band upward conversion circuit.
In the base band signal transmitting device of the base station of the aforementioned conventional mobile communications system, the signal transmission between the amplifier and buffer amplifier is performed via a bus, and signal transmission between the buffer amplifier and base/intermediate frequency band upward conversion circuit is carried out by sectors in analog mode using the separate cable. Accordingly, noise component is introduced during the transmission, resulting in deterioration of calling quality. Furthermore, it is impossible to obtain cutoff characteristic and linear phase characteristic which are required for the mobile communications system because the base band low pass filter is configured of passive elements. Also, the conventional base band signal transmitting device needs large area. Moreover, the CDMA modems are mounted on the same board together with the digital summing device, DAC, amplifier and low pass filter so that the number of CDMA modems which can be mounted on one board is limited.